1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention is a papermaker's fabric of the on-machine-seamable (OMS.RTM.) variety, such as an OMS.RTM. press fabric for the press section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a fibrous cellulosic web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, on a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric during this process, leaving the fibrous cellulosic web on the surface of the forming fabric.
The newly formed cellulosic web proceeds from the forming section to a press section, which includes a series of press nips. The fibrous cellulosic web passes through the press nips supported by a press fabric, or, as is often the case, between two press fabrics. In the press nips, the fibrous cellulosic web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulose fibers in the web to one another to turn the fibrous cellulosic web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a sinuous path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speed. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Referring, for the moment, specifically to press fabrics, it should be recalled that, at one time, press fabrics were supplied only in endless form. This is because a newly formed paper sheet is extremely susceptible to marking in the press nip by any nonuniformity in the press fabric or fabrics. An endless, seamless fabric, such as one produced by the process known as endless weaving, has a uniform structure in both its longitudinal (machine) and transverse (cross-machine) directions. A seam, such as a seam which may be used to close the press fabric into endless form during installation on a paper machine, represents a discontinuity in the uniform structure of the press fabric. The use of a seam, then, greatly increases the likelihood that the paper sheet will be marked in the press nip.
It follows, then, that the seam region of any workable on-machine-seamable (OMS.RTM.) press fabric must behave under load, that is, under compression in a press nip, like the rest of the press fabric, and must have the same permeability to water and to air as the rest of the press fabric, in order to prevent the periodic marking of the paper product being manufactured by the seam region. OMS.RTM. is a registered trademark of Albany International Corp.
Despite the considerable technical obstacles presented by these requirements, it remained highly desirable to develop an on-machine-seamable (OMS.RTM.) press fabric, because of the comparative ease and safety with which it could be installed on the press section. Ultimately, these obstacles were overcome with the development of press fabrics having seams formed by providing seaming loops on the crosswise edges of the two ends of the fabric. The seaming loops themselves are formed by the machine-direction (MD) yarns of the fabric. A seam is formed by bringing the two ends of the press fabric together, by interdigitating the seaming loops at the two ends of the fabric, and by directing a so-called pin, or pintle, through the passage defined by the interdigitated seaming loops to lock the two ends of the fabric together. Needless to say, it is much easier and far less time-consuming to install an OMS.RTM. press fabric, than it is to install an endless press fabric, on a paper machine.
There are several methods for producing a press fabric that can be joined on the paper machine with such a seam. One method is to flat-weave the fabric, in which case the warp yarns are the machine-direction (MD) yarns of the press fabric. To form the seaming loops, the warp ends are woven some distance back into the fabric body in a direction parallel to the warp yarns. Another technique, far more preferable, is a modified form of endless weaving, which normally is used to produce an endless loop of fabric. In modified endless weaving, the weft, or filling, yarns are continuously woven back and forth across the loom, in each passage forming a loop on one of the edges of the fabric being woven by passing around a loop-forming pin. As the weft yarn, or filling yarn, which ultimately becomes the MD yarn in the press fabric, is continuous, the seaming loops obtained in this manner are stronger than any that can be produced by weaving the warp ends back into the ends of a flat-woven fabric. In still another method, a fabric is woven endless, and the endless loop of fabric thereby obtained is flattened and given the form of two fabric layers joined to one another at two widthwise ends of the flattened loop. One or more widthwise yarns are then removed from each of the two widthwise ends to produce a short gap defined by the freed, that is, the newly unwoven portions of, lengthwise yarns at each end. These unwoven portions of the lengthwise yarns are then used as seaming loops when the two widthwise ends are brought together as described above.
Generally, the manufacture of an on-machine-seamable (OMS.RTM.) press fabric includes the attachment of a staple fiber batt to one or both sides thereof. The attachment may be effected by a process called needling (fiber locking) or by hydroentangling, while the OMS.RTM. fabric is joined in endless form. Once the desired amount of staple fiber batt has been attached, the loop-forming pin or pintle is removed to place the OMS.RTM. press fabric into flat form for shipment and eventual installation on a paper machine. At this time, the staple fiber batt must be cut in the vicinity of the seam to completely separate the two ends of the OMS.RTM. press fabric from one another. Often, the staple fiber batt is cut in a manner that enables it to form a flap over the seaming loops when the OMS.RTM. press fabric is rejoined into endless form. In this way, the seam region is practically indistinguishable from the rest of the paper-supporting side of the press fabric.
On the other side, the "roll" side, of the press fabric, however, some staple fiber batt must be removed from the seaming loops to facilitate the later passage of a pintle therethrough. The removal of this generally small amount of staple fiber batt, nevertheless, makes the seam region slightly more permeable to air and water than the rest of the press fabric. This difference in water permeability, or flow resistance, perhaps ever so slight, is enough to cause sheet marking in some situations.
Several approaches to solve this problem have been taken. One approach involves the use of stuffer yarns with the pintle when the OMS.RTM. press fabric is being joined into endless form on the paper machine. In another approach, an OMS.RTM. press fabric comprises two on-machine-seamable base fabrics, one fitting inside the loop of the other, the two base fabrics being laminated to one another during the needling process. The seam regions of the inner and outer base fabrics are offset slightly with respect to one another, so that the seam region of each will coincide with a non-seam region of the other. Once the desired amount of staple fiber batt has been attached to the inner and/or outer surfaces of the laminated base fabrics, the loop-forming pin or pintle of each on-machine-seamable base fabric is removed to place the OMS.RTM. press fabric into flat form for shipment and eventual installation on a papermachine. At this time, the staple fiber batt must be cut in the vicinity of the seam in the outer of the two on-machine-seamable base fabrics to completely separate the two ends of the OMS.RTM. press fabric from one another. As above, the staple fiber batt may be cut in a manner that enables it to form a flap over the seaming loops when the OMS.RTM. press fabric is rejoined into endless form. Some of the staple fiber batt must also be removed from the seaming loops of both the inner and outer on-machine-seamable base fabrics to facilitate the later passage of pintles therethrough.
In yet another approach, disclosed in U.S. Pat. Nos. 5,476,123 and 5,531,251 to Rydin, one or more extra CD yarns are woven with the seaming loops of at least one end of a base fabric of an on-machine-seamable press fabric. The extra yarn or yarns are woven only with those portions of the seaming loops that are on one side of the fabric, that side preferably being the paper-supporting side. The extra CD yarn or yarns form an extension of the CD yarn system of the base fabric at the seaming loop or loops, conforming the seam region more closely to the rest of the base fabric, so that staple fiber batt will be better anchored to the seam region and so that the possibility of sheet marking by the seam region will be reduced.
The last approach has also been combined with that set forth immediately above in OMS.RTM. press fabrics having two on-machine-seamable base fabrics laminated to one another during the needling process, one base fabric again fitting inside the loop of the other. In this hybrid approach, the outer base fabric incorporates the invention disclosed in U.S. Pat. Nos. 5,476,123 and 5,531,251, the teachings of both of which are incorporated herein by reference.
All of these approaches are designed to compensate for the differences between the water permeability or flow resistance of the seam region of an OMS.RTM. press fabric and the rest or body of the press fabric. However, none of these approaches has yielded completely satisfactory results for all press types and positions and for all paper grades.
The present invention represents an alternative approach toward solving this problem.